1. Field of the Invention
This invention relates to an external combustion engine for displacing a liquid-phase portion of a working medium by evaporation and condensation of the working medium and outputting by converting the displacement of the liquid-phase portion of the working medium into mechanical energy.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2005-330885 discloses a external combustion engine, in which a container having a working medium adapted to flow in a liquid phase is provided with at least an evaporator for heating and evaporating part of the working medium in a liquid phase and at least a condenser for cooling and condensing the working medium evaporated in the evaporator. In this configuration, the liquid-phase portion of the working medium is displaced by the evaporation and condensation thereof and this displacement of the liquid-phase portion of the working medium is converted into mechanical energy which is retrieved from an output unit.
In this technique, the portion of the container near the output unit is configured of a single collecting pipe, while the portion of the container formed with an evaporator and condenser is configured of a multiplicity of branch pipes thereby increasing the heat transmission area of the evaporator and condenser. As a result, the heating (evaporation performance) and cooling performance (condensation performance) of the working medium are improved for an improved output of the external combustion engine.
According to this technique, the portion of the multiplicity of the branch pipes formed with the evaporator is arranged in the flow of a high-temperature gas to heat the working medium with the high-temperature gas as a heat source.
Also, according to this technique, the multiplicity of the branch pipes are arranged both in the direction along the flow of the high-temperature gas and in the direction perpendicular to the direction of the high-temperature gas flow. In other words, the multiplicity of the branch pipes are arranged in a grid pattern to thereby prevent the multiplicity of the branch pipes from making the container bulky.
However, in the techniques described above, the high-temperature gas flowing from the upstream to the downstream side is deprived of heat by the evaporator of the multiplicity of the branch pipes and decreases in temperature. As a result, the more upstream the evaporator of the branching pipes arranged in the high-temperature gas is, the greater the heat exchange amount, and vice versa.
Consequently, the working medium, in the evaporator of the branching pipes upstream in the high-temperature gas flow is sufficiently heated and evaporated at the boiling point. However, the working medium in the evaporator of the branch pipes downstream in the high-temperature gas flow is not sufficiently heated and often fails to reach the boiling point.
In the branch pipes downstream in the high-temperature gas flow, the displacement amount of the liquid-phase portion of the working medium is decreased, resulting in a smaller output. Specifically, if the working medium fails to reach the boiling point after being heated, the corresponding heat loss deteriorates the efficiency of the external combustion engine. This problem of deteriorated efficiency due to the heat loss occurs also in the case where two as well as a multiplicity of branch pipes are provided in this engine.
In the case where one heat generating unit is arranged in the neighborhood of the container and where the evaporator is heated by the heat generated by the one heat generating unit, the working medium is heated sufficiently in the branch pipes in the neighborhood of the one heat generating unit, while the working medium cannot be sufficiently heated in the branch pipes far from the one heat generating unit, thereby posing a similar problem of efficiency deterioration due to heat loss.